1. Field of the Invention
The present invention relates in general to a chemical mechanical polishing (CMP) process. More particularly, the present invention relates to a multistage chemical mechanical polishing process for forming shallow trench isolation structure capable of reducing the cost, simplifying the manufacturing process and increasing the throughput thereof.
2. Related Art of the Invention
The conventional localized oxidation isolation (LOCOS) structure is being gradually replaced by the shallow trench isolation (STI) structure because during the localized oxidation isolation (LOCOS) process, a bird's beak effect is generated rendering the surface of the products non-uniform. In general, it is well recognized in the art that for a semiconductor process having a line width less than 0.25 μcm, the shallow trench isolation (STI) is preferably used because STI not only eliminates the bird's beak effect but also occupy smaller space compared LOCOS. Accordingly, a higher integration of the circuits can be achieved by utilizing STI structure.
FIG. 1A to FIG. 1D are cross-sectional views illustrating a conventional process of forming a shallow trench isolation (STI) structure. First, referring to FIG. 1A, a substrate 102 is provided, and a pad oxide layer 104 and a silicon nitride layer 106 are sequentially formed on the substrate 102. Next, the pad oxide layer 104 and the silicon nitride layer 106 are patterned by performing a lithography and etching processes to form a plurality of trenches in the silicon nitride layer 106, the pad oxide layer and the substrate 102 as shown in FIG. 1B. As shown in FIG. 1B, the pitch, line width and density of the trenches in the dense area 112 are different from that of the isolation area 114. Next, as shown in FIG. 1C, an oxide liner 122 is formed covering the side wall and the bottom of the trenches in order to repair the damage caused by the etching process. The method of forming the oxide liner 122 includes, for example, a thermal oxidation. Next, as shown in FIG. 1D, an oxide layer 132 is formed over the resulting structure filling the trenches. The method of forming the oxide layer 132 includes, for example, a chemical vapor deposition (CVD) method or a high density plasma (HDP) chemical vapor deposition (CVD) method.
With the rapid advancement of the semiconductor manufacturing process, the line width is gradually reduced from 0.18 μm, 0.13 μm to 0.10 μm or less, for example, 90 nm or sub-90 nm. When the line width is shrunk to such a level, in general, the high density plasma chemical vapor deposition (HDP-CVD) method is still useful as a gap-fill process for forming the shallow trench isolation (STI) structure. The fill material used in the HDP-CVD method is silicon dioxide. However, when the line width is shrunk, the aspect ratio (AR) of depth to width of the trench will correspondingly increase making that gap filling process difficult. Consequently, voids are easily formed within the trench, and thereby rendering the gap-fill process very challenging and complex. Thus, in the gap-fill process of the shallow trench isolation (STI) structure of 90 nm and sub-90 nm level, a deposition-etching-deposition (DED) gap-fill process by using a multi-step gap-fill process is developed, in order to achieve void-free gap-fill of a high aspect ratio (AR) trench to form void-free shallow trench isolation (STI) structure. The etching process can be a dry etch or a wet etch process.
However, the disadvantage of the multi-step gap-fill process described above is that the thickness of the overburden over the shallow trench isolation structure is increased drastically, and the difference of the overburden thickness in the isolation area and the dense area (the iso/dense thickness uniformity) is also increased. For example, in the 0.13 μm process, a single deposition step is used for forming the oxide layer 132, the resulting overburden thickness is about 150 nm, and the iso/dense thickness uniformity is in a range of about 20 nm. However, in the 90 nm process, when the above-described deposition-etching-deposition (DED) gap-fill process is performed, the resulting overburden thickness is about 400 nm, and the iso/dense thickness uniformity is in a range of about 300 nm. Therefore, when resulting structure is planarized by performing a chemical mechanical polishing (CMP) process, the overpolishing of the oxide layer over the isolation area 114 easily occurs.
The chemical mechanical polishing (CMP) process, in general, has the advantages of low cost, high removal rate and high uniformity efficiency. Taking a silica based shallow trench isolation structure, the removal rate (RR) is more than 250 nm/min. However, the disadvantage is that the selectivity of oxide to nitride is low and therefore the insufficient polishing or over-polishing of the oxide layer occurs, and thus requires an additional external process using a reserve mask (RM) to resolve this problem.
In order to overcome the disadvantages of the CMP process described above, a chemical mechanical polishing (CMP) method using a high selectivity slurry (HSS) or a fixed abrasive (FA) pad and without requiring the use of reserve mask is developed recently. In general, the major polishing components in the HSS slurry include cerium oxide (CeO2). The major polishing particles of the fixed abrasive (FA) polishing method may also include cerium oxide (CeO2). Although a higher oxide to nitride selectivity is obtained in the improved chemical mechanical polishing (CMP) method described above, however, this improved CMP method still has certain disadvantages, such as a higher cost, a low removal rate and non-uniformity. For example, it is preferable that the difference of thickness uniformity of the polished layer is in a range of about 300 nm. Moreover, for example, the removal rate of the high selectivity slurry is generally in a range of about 60 nm/min, the removal rate of the fixed abrasive (FA) is about 150 nm/min, and both of the removal rates are less than that of the conventional CMP method. Therefore, in the post shallow trench isolation chemical mechanical polishing (post STI-CMP) process, if only the high selectivity slurry or the fixed abrasive (FA) pad is used, the performance of the planarization is prolonged compared to the conventional CMP method and therefore not acceptable. Moreover, the residues of the oxide layer are easily generated. Therefore, a CMP polishing method that can reduce the cost, simply the process, and increase the throughput is highly desirable.